Conventionally, a tire as a finished product is subjected to tire testing to determine the quality thereof by measuring uniformity or the like (uniformity check). For example, testing for uniformity measurement on a tire for passenger car is basically performed according to the following procedure by use of a testing device shown in Patent Literature 1.
A tire testing device described in Patent Literature 1 includes a pair of upper and lower rims, a factory air source that outputs compressed air to be supplied to a tire held between both the rims, and a pneumatic circuit that regulates the pressure of the compressed air output from the factory air source and then supplies the resulting compressed air to the tire. The tire testing is performed after the tire is inflated by this pneumatic circuit.
The pneumatic circuit includes two systems of piping branched from each other at the middle, and a changeover valve. One piping is piping of bead seat system for inflating the tire in a short time to fit the tire to the rims, and the other is piping of testing system used for tire testing. The changeover valve switches, for a pipe connected to the tire, between the piping of bead seat system and the piping of testing system, whereby the tire can be inflated by use of these two systems of piping routes.
Using this tire testing device, the tire testing is performed as follows. A tire carried from the upstream of an inspection line is set on the rims. The tire is then inflated in a short time by use of the piping of bead seat system. The air pressure of compressed air to be supplied to the tire through the piping of bead seat system is generally set to a pressure (e.g., about 0.4 MPa) higher than a testing air pressure that is a pressure used in tire testing, and the internal pressure of the tire is maintained at this air pressure for about 1 second including the pressure rise time.
The changeover valve is then operated to switch, for the flow path of compressed air, from the piping of bead seat system to the piping of testing system. A pressure regulating valve is provided at the middle of the piping of testing system. The pressure regulating valve decompresses high-pressure compressed air to the testing air pressure (e.g., about 0.2 MPa). The thus-decompressed compressed air is supplied into the tire through the piping of testing system, whereby the air pressure inside the tire is regulated to the above-mentioned testing air pressure. A drum including a load measuring instrument is pressed onto the tire having an internal pressure thus maintained at the testing air pressure, and the load measuring instrument measures a repulsive force generated in the tire at that time. Due to this, the uniformity of the tire is measured.
As the above-mentioned pressure regulating valve, a servo-type pressure regulator is often used as shown in Patent Literature 2.
In the above-mentioned tire testing, the variation of the air pressure inside the tire during testing is known to seriously affect a measurement result of uniformity. Therefore, it is important to further accurately maintain the air pressure inside the tire at a fixed testing air pressure in order not to market a defective product, nor to falsely determine a non-defective product as defective.
However, actual tire testing frequently involves change of air pressure, and the air pressure decreases or increases in rare cases. Such a change of the air pressure inside the tire is as small as about 0.5 kPa in some cases and as large as about 1 kPa in other cases. However, even a change of air pressure as small as about 0.5 kPa seriously affects the measurement result of uniformity. For example, when the repetitive stability of the testing device is confirmed, a same tire has to be repetitively tested. However, if the air pressure inside the tire changes in each testing, the repetitive stability of the testing device cannot be surely determined since the measurement result is differed in every measurement even if the same tire is used, leading to the difficulty to secure the quality for testing device/testing line.
It is difficult to regulate a minor change of air pressure as described above by use of such a pressure regulating valve generally used in the tire testing device shown in Patent Literature 1. The pressure regulating range of such a general pressure regulating valve is about 1.0 MPa, and the pressure regulating accuracy thereof is ±0.1% or about 1 kPa at most. The pressure regulating valve having pressure regulating accuracy of only about 1 kPa can never regulate the air pressure inside the tire that varies at a level of about 0.5 kPa during tire testing.
The servo-type pressure regulating valve disclosed in Patent Literature 2 is excellent in pressure regulating accuracy but low in responsiveness. Therefore, this valve can respond to gentle and steady variation in air pressure but cannot timely regulate the air pressure inside the tire within a tire testing time of only about 1 second. Namely, it is difficult to regulate the air pressure inside the tire, which varies during such short-time testing, by use of the servo-type pressure regulating valve as shown in Patent Literature 2. Further, the servo-type pressure regulating valve is expensive, and the use thereof leads to a steep rise in price of the tire testing device.